Proatlas
The proatlas is a paired bone in the skeleton of some living and many extinct terrestrial vertebrates (tetrapods) that occurs between the skull and the atlas, the first cervical vertebra. It ossifies endochondrally.[1]
A number of different interpretations have been made of the proatlas.[2][3] The most common interpretation is that it is the vestigial neural arch of a vertebra that is otherwise fully incorporated into their skull, but the development shows some differences from other vertebrae that present difficulties for this hypothesis.[1] The proatlas of dinosaurs was first recognized by Othniel Marsh, who initially termed them the "post-occipital bones",[4] but their homology with the proatlas of other reptiles was subsequently recognized by Charles W. Gilmore.[5]
The proatlas was not present in early finned tetrapodomorphs, but are present in the limbed stem-tetrapod Greererpeton. Proatlases were probably widely present in early tetrapods, with records in aïstopods, temnospondyls, embolomeres and seymouriamorphs. They were widely present among non-mammalian synapsids, but were lost in the mammaliaform ancestors of mammals. They were also widely present among extinct reptiles, including dinosaurs.[6] The proatlas is absent as a separate element in birds, perhaps as a consequence of fusion with the braincase, with Archaeopteryx the closest relative of modern birds known to retain a separate proatlas.[7] They have also been lost in modern amphibians (Lissamphibia), turtles and squamates (lizards and snakes), but are present in the stem-squamate Huehuecuetzpalli.[6] They are present in the tuatara,[6] though they have not yet been identified in extinct rhynchocephalians.[8] They are also present in living crocodilians, in which the left and right proatlases fuse into a single V-shaped midline element.[1][3] The proatlas plays a role in the circulation of cerebrospinal fluid in crocodylians.[9]
A proatlas can occur pathologically in humans.[10]
References
[edit]- 1 2 3 Korneisel, Dana E.; Vice, Rebekah; Maddin, Hillary C. (2021-11-30). "Anatomy and development of skull–neck boundary structures in the skeleton of the extant crocodylian Alligator mississippiensis". The Anatomical Record. 305 (10). doi:10.1002/ar.24834. eISSN 1932-8494. ISSN 1932-8486.
- ↑ Baur, G. (1886). "The proatlas, atlas and axis of the Crocodilia". The American Naturalist. 20 (3): 288–293. doi:10.1086/274205. ISSN 0003-0147.
- 1 2 Mook, Charles C. (1921). "Notes on the postcranial skeleton in the Crocodilia". Bulletin of the American Museum of Natural History. 44: 67–100.
- ↑ Marsh, O. C. (1883). "Principal characters of American Jurassic dinosaurs. Part VI: restoration of Brontosaurus" (PDF). American Journal of Science. 3. 26 (152): 81–85. doi:10.2475/ajs.s3-26.152.81. ISSN 0002-9599.
- ↑ Gilmore, Charles W. (1907). "The type of the Jurassic reptile Morosaurus agilis redescribed, with a note on Camptosaurus". Proceedings of the United States National Museum. 32 (1519): 151–165. doi:10.5479/si.00963801.32-1519.151. ISSN 0096-3801.
- 1 2 3 Korneisel, Dana E.; Maddin, Hillary C. (2025-07-22). "Review of the tetrapod skull–neck boundary: implications for the evolution of the atlas–axis complex". Biological Reviews. doi:10.1111/brv.70053. PMC 12586291.
- ↑ O’Connor, Jingmai; Clark, Alexander; Kuo, Pei-Chen; Kiat, Yosef; Fabbri, Matteo; Shinya, Akiko; Van Beek, Constance; Lu, Jing; Wang, Min; Hu, Han (2025-05-14). "Chicago Archaeopteryx informs on the early evolution of the avian bauplan". Nature. doi:10.1038/s41586-025-08912-4. eISSN 1476-4687. ISSN 0028-0836.
- ↑ Beccari, Victor; Jones, Marc E H; Villa, Andrea; Martino, Roberta; Regnault, Sophie; Glaw, Frank; Rauhut, Oliver W M (2026-04-01). "The axial skeleton of the tuatara (Rhynchocephalia: Sphenodon ): insights on intraspecific variability, ontogeny, sexual dimorphism, and remarks on fossil taxa". Zoological Journal of the Linnean Society. 206 (4). doi:10.1093/zoolinnean/zlaf135. ISSN 0024-4082.
- ↑ Swords, Annelise; Cramberg, Michael; Parker, Seth; Scott, Anchal; Sopko, Stephanie; Taylor, Ethan; Young, Bruce A. (2024). "The Crocodylian proatlas functions to redistribute venous blood and cerebrospinal fluid". Journal of Morphology. 285 (3): –21683. doi:10.1002/jmor.21683. ISSN 1097-4687. PMID 38424675.
- ↑ Muhleman, Mitchel; Charran, Ordessia; Matusz, Petru; Shoja, Mohammadali M.; Tubbs, R. Shane; Loukas, Marios (2012). "The proatlas: a comprehensive review with clinical implications". Child's Nervous System. 28 (3): 349–356. doi:10.1007/s00381-012-1698-8. eISSN 1433-0350. ISSN 0256-7040.